Normally succulents are not the sort of plants that come to mind when one discusses invasive species. Most think of slow growing, solitary plants that either sit in a pot on a window sill, or struggle along in a desert garden, often grown like a living piece of art (which indeed many succulents are). But after years of learning about, collecting and growing succulents of many different kinds, I have discovered some are not such innocuous plants. Some succulent species are quite aggressive in their nature spreading rapidly or sneakily throughout their environment, often successfully out-competing the other garden flora, in an attempt to take over and reproduce their genetic material in a way to ensure its ultimate survival.
Agaves- most agave species sucker (offset) so one could say all are potentially invasive. However, most of the suckering species tend to produce offsets right at the base of the mother plant making their invasiveness a nonissue except over geologic time. But a few species are particularly annoying in my garden and most of these are among the larger, spinier and most commonly available (therefore lower down on the collectors ‘want list'). Agave americana is the most aggressive of all, spreading all over the garden, often over a dozen feet from the mother plant. It is tough as nails and a very fast grower, so plant this one in the garden only if you really want it to take over, or you have tons of room to fill (makes a good plant for decorating southern California highways). Agave angustifolia is another baddie in my opinion, traveling some distance and is particularly spiny and difficult to eradicate once it's gotten its roots entrenched in the soil. And Agave sisalana, a monster of a species, is another remote offsetter that I am still, and perhaps always will be battling in my garden. Fortunately this one has no teeth along the leaves so it is a bit easier to get a handle on.... But still has a viscous terminal spine.
Agave americana Agave angustifolia
Agave sisilana (left) and Agave tequiliana (right) are also moderate invasive and large plants, adding to their invasive potential
Aloes are one of my favorite genera and I like most of them. But a few are somewhat invasive locally. The spotted aloes, primarily Aloe greatheadii, Aloe greenii and Aloe maculata can pop up here and there, sometimes several feet from where another plant was growing. The problem for me is my limited space and my prejudice against most of these look-alike species. Aloe greenii is the worst, though. Give this one some water and it will take off, not only obtaining an obscene size for a spotted aloe, but it has the green-spotted aloe's razor sharp teeth and this one grows really fast and spreads in a hurry. Risking life and limb (well, really only limb), I have to gingerly step through my garden where this plant is growing and try to rip out the suckers, using thick gloves and often accidentally yanking up other aloes which I want to keep, as this ones humongous roots grab on to all it can while I tear it out of the ground. Sounds worse than it is really, but it is an invasive species in my yard.
Aloe greenii as a seedling on left and just 2 years later a huge plant 3' across with razor teeth and starting to show up all over
Ice plants- there are dozens of mesembs (members of the family Aizoacea) that are collectively called Ice Plants that are grown specifically because they can swarm and create a relatively large area of ground cover. These are not really that difficult to control as they are weak-rooted and spineless plants, requiring one to just pull them up when they get ‘out of bounds'. Still, they wrap over and around other plants if given the opportunity and may take the other plants out with them when you are tearing these ice plants out of the garden. Many produce fantastic areas of color at certain times of the year so are tempting to put in the garden wherever there's room, but think ahead what ripping them out will be like.
First photo is of Drosanthemum floribundum, and 'ice plant'; my own Delosperma species taking over my yard in second photo
Lampranthus on left and unknown ice plant on right, both swarming groundcovers
There are a few Euphorbias I would characterize as invasive, though they are certainly the exceptions of the genus. Some species, like the Spurges, are very invasive, but I know enough not be letting those things lose in my yard. I prefer solitary to shrubby plants, not fields of the same thing all ove the place. Euphorbia xantii, a thin-stemmed, showy plant with hundreds of white and pink flowers in winter, tends to magically appear everywhere once established in the garden. It is not difficult to pull up the new seedlings that keep popping up, but then one tends to get the noxious Euphorbia latex all over one's skin and clothing. Euphorbia lambii, another prolific seed-thrower, also tends to show up in large numbers some distance from the mother plant.
Euphorbia xantii can be a spectacular as a garden plant the right time of year, but be careful on new seedlings that sprout up all over within about a fifteen foot radius or will be up their necks in these shrubs
- Euphorbia lambii is another shrub/tree Euphorbia that sort of shoots its seeds all over the garden
Probably the king or queen of all succulent invasive species are the Kalanchoes that drop little plantlets from their leaves. Most of these at some time were classified in the genus Bryophyllum, but seem to end up back in Kalanchoe every time an attempt to reclassify them occurs. Their common name is aptly Mother of Thousands and this is no exaggeration (if anything, it might be an under-exaggeration). This plant, once it makes its way into your collection, is often there to stay. Sometimes it shows up unannounced along with another purchase, or just seemingly out of thin air. They are easy to pull up, but often end up growing in so many difficult-to-get-to cracks and crevices in the garden, or among the very spiny plants, that they are impossible to hunt them all down and eliminate them all. At least they are somewhat ornamental in appearance.
Kalanchoe diagremontiana (Mother of Thousands) looks nice... but close in spection of the leaves as in the photo on the right (by Todd Boland) shows the miniature plantlets that fall and grow everywhere.
My own Mother of Thousands plant, showing all the little babies 'appearing' out of nowhere below in pot; a pink form showing the huge number of plantlets on the leaves (right), though actually this form is not invasive as the little plantlets cannot root (clever bioengineering, or lucky cultivar)
Kalanchoe delagoensis, a closely related species, also has the common name Mother of Thousands and for good reason. Once this is in the garden, it will show up everywhere for years
Ornithogalum longibracteatum, aka Pregnant Onion, is another truly efficient invasive species. This weird bulb/succulent looks a lot like a giant green onion and produces nice, long flower stalks that produce a lot of seeds. But it also forms dozens of clones of itself along the surface of its body that somehow show up everywhere. They only have to touch the ground and they will grow happily from wherever they end up. Eventually they all grow into these large green bulbs pushing all other plants out of the way. They are relatively easy to pull up, but they have fairly big root-balls and most surrounding plants tend to come up with them, requiring one to replant the garden whenever these are removed.
Pregnant Onion in a pot (left) still manages to get its babies (seen clinging to 'mom' in right photo) all over the garden, even without being planted in it
Some ground cover Sedums and Senecios also are invasive. The Senecios tend to look like Ice Plants, though most are bluish and quite ornamental (but still start growing over everything and smothering them). The sedums are less swarming, but still spread slowing and end up everywhere (notably Sedum x rubrotinctum). All these plants are pretty simple to remove.
Sedum x rubrotinctum grown as a groundcover (left), but showing in my garden here and there (right)
One of the blue Senecios (called Blue Chalk) growing as a groundcover (left), but invading my garden, swarming over everything in its path... easy to pull up, though
I don't know if any of these plants would qualify as really that economically or ecologically damaging, so no one would likely consider them serious invasive species. I prefer to call them annoyingly invasive. But someone's invasive weedy succulent is always someone else's pride and joy, so it's hard to be too critical of any of these plants.